It is known that antenna performance is dependent upon the size, shape and material composition of the constituent antenna elements, as well as the relationship between certain antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These relationships determine several antenna operational parameters, including input impedance, gain, directivity, signal polarization and radiation pattern.
Generally, an operable antenna should have a minimum physical antenna dimension approximately equal to a half wavelength (or a quarter wavelength above a ground plane) (or a multiple thereof) of the operating frequency to limit energy dissipated in resistive losses and maximize transmitted or received energy. A quarter wavelength antenna (or multiples of a quarter wavelength) operative above a ground plane exhibits properties similar to a half wavelength antenna. Designers of communications products prefer an efficient antenna that is capable of wide bandwidth and/or multiple frequency band operation, electrically matched to the transmitting and receiving components of the communications system, and operable in multiple modes (e.g., selectable signal polarizations and selectable radiation patterns). Quarter wavelength and half wavelength antennas are the most commonly used.
The half-wavelength dipole antenna finds use in many applications. The radiation pattern is the familiar donut shape with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. Frequency bands of interest for certain communications devices are 1710 to 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz, and 2.68 inches long at 2200 MHz. The typical gain is about 2.15 dBi.
The quarter-wavelength monopole antenna disposed above a ground plane is derived from the half-wavelength dipole. The physical antenna length is a quarter wavelength, but interaction of the electromagnetic energy with the ground plane causes the antenna to exhibit half wavelength dipole properties. Thus, the radiation pattern for a monopole antenna above a ground plane is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi.
Given the advantageous performance of quarter and half-wavelength antennas, conventional antennas are typically constructed so that the antenna length is on the order of a quarter wavelength of the radiating frequency and the antenna is operated over a ground plane or the antenna length is a half wavelength without employing a ground plane. These dimensions allow the antenna to be easily excited and operated at or near a resonant frequency (where the resonant frequency (f) is determined according to the equation c=λf, where c is the speed of light and λ is the wavelength of the electromagnetic radiation). Half and quarter wavelength antennas limit energy dissipated in resistive losses and maximize the transmitted energy. As the operating frequency increases/decreases, the operating wavelength decreases/increases and the antenna element dimensions proportionally decrease/increase. In particular, as the resonant frequency of the received or transmitted signal decreases, the dimensions of the quarter-wavelength and half-wavelength antenna proportionally increase. The resulting larger antenna, even at a quarter wavelength, may not be suitable for use with certain communications devices, especially portable and personal communications devices intended to be carried by a user. Such antennas can protrude from the communications device and thus are susceptible to breakage
The burgeoning growth of wireless communications devices and systems has created a substantial need for physically smaller, less obtrusive and more efficient antennas capable of wide bandwidth or multiple frequency-band operation. It is also desired that the antennas operate in multiple modes (i.e., selectable radiation patterns or selectable signal polarizations). For example, operation in multiple frequency bands may be required for operation of the communications device with multiple communications systems, such as a cellular telephone system and a cordless telephone system. Operation of the device in multiple countries also requires multiple frequency band operation since communications frequencies are not commonly assigned among countries.
Smaller packaging of state-of-the-art communications devices, such as personal handsets, does not provide sufficient space for the conventional quarter and half wavelength antenna elements. Generally, it is not considered feasible to utilize one antenna for each operating frequency or to include multiple matching circuits to provide proper resonant frequency operation at several different frequencies for a single antenna. Thus physically smaller antennas operating in frequency bands of interest and providing the other desired antenna properties (input impedance, radiation pattern, signal polarizations, etc.) are especially sought after.
As is known to those skilled in the art, there is a direct relationship between physical antenna size and antenna gain for a single-element antenna, according to the relationship: gain=(βR)^2+2βR, where R is the radius of the sphere containing the antenna and β is the propagation factor or phase constant. Increased gain thus requires a physically larger antenna, while users continue to demand physically smaller antennas. As a further design constraint, to simplify the system design and strive for minimum cost, equipment designers and system operators prefer to utilize antennas capable of efficient multi-band and/or wide bandwidth operation, thus permitting the communications device to access various wireless services operating within different frequency bands or services operating over wide bandwidths. Finally, gain is limited by the known relationship between the antenna operating frequency and the effective antenna length (expressed in wavelengths). That is, the antenna gain is constant for all quarter-wavelength antennas of a specific geometry i.e., at that operating frequency where the effective antenna length is a quarter of the operating frequency wavelength.
To overcome the size limitations of handset and personal communications devices, antenna designers have turned to the use of so-called slow wave structures where the structure's physical dimensions are not equal to the effective electrical dimensions. Recall that the effective antenna dimensions should be on the order of a half wavelength (or a quarter wavelength above a ground plane) to achieve the beneficial radiating and low loss properties discussed above. Generally, a slow-wave structure is defined as one for which the phase velocity of the traveling wave is less than the free space velocity of light. The wave velocity is the product of the wavelength and the frequency and takes into account the material permittivity and permeability, i.e., c/((sqrt(εr)sqrt(μr))=λf. Since the frequency remains unchanged during propagation through a slow wave structure, if the wave travels slower (i.e., the phase velocity is lower) than the speed of light, the wavelength within the structure is lower than the free space wavelength. The slow-wave structure de-couples the conventional relationship between physical length, resonant frequency and wavelength.
Since the phase velocity of a wave propagating in a slow-wave structure is less than the free space velocity of light, the effective electrical length of these structures is greater than the effective electrical length of a structure propagating a wave at the speed of light. The resulting resonant frequency for the slow-wave structure is correspondingly increased. Thus if two structures are to operate at the same resonant frequency, as a half-wave dipole, for instance, then the structure propagating a slow wave will be physically smaller than the structure propagating a wave at the speed of light. Such slow wave structures can be used as antenna elements or as antenna radiating structures.